1. Field of the Invention
The present invention relates to an exposure apparatus to be used in a photolithography process for manufacturing a semiconductor device, a liquid crystal display device, a thin film magnetic head, etc. and more particularly to a scanning type exposure apparatus for exposing a pattern of a mask (or reticle) to a photosensitive substrate by shifting the mask and the photosensitive substrate synchronously.
2. Related Background Art
In a photolithography process for manufacturing a semiconductor device, a projection exposure apparatus is used wherein the pattern of a photomask or a reticle (hereinafter referred to the reticle) is transferred via a projection optical system to a semiconductor wafer (or a glass plate, etc.) coated with a photosensitive material (photoresist). Presently, reduction projection type exposure apparatuses (steppers) of a step-and-repeat system disclosed in e.g., U.S. Pat. Nos. 4,677,301 and 4,962,318 have been widely used. As illumination for exposure, emission lines (i-line and the like) from a mercury lamp, a KrF or ArF excimer laser or a higher harmonic such as of a metal vapor laser or a YAG laser is used.
In projection exposure apparatuses as disclosed in e.g., U.S. Pat. Nos. 4,712,910 and 4,884,101, a shutter is utilized to open and close the path of light from a light source thereby to control the amount of exposure. That is, the amount of exposure imparted to a wafer is controlled to an optimum value corresponding to the sensitivity of the photoresist of the wafer. Especially in projection type exposure apparatuses with pulsed laser light sources such as of an excimer laser or the like, as disclosed in, e.g., U.S. Pat. Nos. 4,970,546, 5,097,291 and 5,191,374, an amount of energy per pulse is set to a predetermined value thereby to control the amount of exposure.
Recently, as semiconductors become large in size and minute in structure, it is required to enlarge the image field of the projection optical system and to improve the resolution thereof. However, it is extremely difficult to obtain both the high resolution and the large image field in the projection optical system from the viewpoint of design and manufacture. Therefore, as disclosed in, e.g., U.S. Pat. Nos. 4,747,678, 4,924,257 and 5,194,893, scanning type projection exposure apparatuses are paid attention in which only a local area of a reticle is illuminated and the reticle and a wafer are shifted synchronously to expose the pattern of the reticle to the wafer. In such scanning type exposure apparatuses, even though the image field of a projection optical system is small, it is possible to exposure a pattern with a large area to the wafer and to improve the resolution of the projection optical system comparatively easily.
However, if the conventional exposure control method is applied to such scanning type exposure apparatuses, the amount of exposure to the wafer cannot be controlled to an optimum value corresponding to the sensitivity of the photoresist. That is, in a scanning type exposure apparatus with a light source emitting continuous light such as of i-lines, even though only a time for opening a shutter is controlled as in U.S. Pat. No. 4,712,910, an optimum amount of exposure cannot be imparted to the wafer. Also, when the sensitivity of the photoresist is changed, the amount of exposure cannot be controlled properly in accordance with the change. Further, in a scanning type exposure apparatus with a light source emitting a light beam such as an excimer laser, etc., there is a chance that the number of light beams illuminating a wafer is different in various positions on the wafer in accordance with the relationship between the rate of movement of the wafer and the timing of emissions of light beams. Namely, there is a change that unevenness of the amount of light occurs.
It is therefore a first object of the present invention to provide a scanning type exposure apparatus in which even though the pattern of a reticle is scanned and exposed to a photosensitive substrate by the use of a light source for emitting continuous light, an optimum amount of exposure can be imparted to the photosensitive substrate in accordance with the sensitivity thereof without lowering the throughput and incurring unevenness of illuminance.
It is a second object of the present invention to provide a scanning type exposure apparatus in which even though the pattern of a reticle is scanned and exposed to a photosensitive substrate by use of a light source for emitting laser lights, an optimum amount of exposure can be imparted to the photosensitive substrate without causing unevenness of the quantity of light.
Therefore, in order to achieve the first object, a first apparatus of the present invention has a light source for emitting continuous light, an illumination optical system for illuminating a local area on a mask with light from the light source and a projection optical system for projecting the image of the pattern of the mask within the local area to a photosensitive substrate with a photosensitive material applied thereon and, the pattern of the mask is scanned and exposed on the sensitive substrate by synchronously shifting the mask and the photosensitive substrate in a predetermined scanning direction perpendicular to an optical axis of the projection optical system. The first apparatus further has an adjusting device for adjusting the intensity of the light to be incident on the substrate and a control device for controlling the adjusting device in accordance with the sensitivity characteristic of the photosensitive material, the speed of the substrate and the width of a projection area of the pattern of the mask by the projection optical system in the scanning direction. Therefore, even though the sensitivity characteristic of the photosensitive material is changed, the intensity of the light is changed accordingly, so that an optimum amount of exposure can be imparted to the substrate. In particular, when the photosensitive material has a low sensitivity, the intensity of the light is increased, so that the speed of the substrate can be maintained to an upper limit (the maximum speed of the substrate stage). Therefore, the lowering of the throughput can be prevented. On the other hand, when the photosensitive material has a high sensitivity, even though the speed of the substrate reaches the upper limit (the maximum speed of the substrate stage), the intensity of the light is decreased, so that an optimum amount of exposure can be imparted to the substrate.
Also, in order to achieve the first object of the present invention, a second apparatus has a light source for emitting continuous light, an illumination optical system for illuminating a local area on a mask with the light from the light source and a projection optical system for projecting the image of a pattern on the mask within the local area to a substrate with a photosensitive material applied thereto. And, the image of the pattern of the mask is scanned and exposed on the substrate by synchronously shifting the mask and the substrate in a predetermined scanning direction perpendicular to the optical axis of the projection optical system. The second apparatus further has an optical member for varying the width of the local area on the mask in the scanning direction and a control device for controlling the optical member in accordance with the intensity of the light to be incident on the substrate, the sensitivity characteristic of the photosensitive material and the speed of the substrate. Therefore, even though the sensitivity characteristic is changed, an optimum amount of exposure can be imparted to the substrate, as the width of the projection area of the pattern of the mask by the projection optical system in the scanning direction is changed. In particular, when the photosensitive material has a low sensitivity, the width of the local illumination area on the mask in the scanning direction is enlarged, so that the speed of the substrate can be maintained to an upper limit (the maximum speed of the substrate stage). Therefore, the lowering of the throughput can be prevented. On the other hand, when the photosensitive material has a high sensitivity, even though the speed of the substrate reaches an upper limit (the maximum speed of the substrate stage), the width of the local illumination area is narrowed, so that an optimum amount of exposure can be imparted to the substrate.
Further, the second apparatus may be provided with a detecting device for detecting the intensity of light to be incident on the substrate and the control device may control the adjusting device in accordance with the output of the detecting device. In this case, even though the intensity (illuminance) of the light is changed with the passage of time, the width of the local illumination area in the scanning direction can be changed in accordance with the change of the intensity, whereby an optimum amount of exposure can be imparted to the substrate.
In the apparatuses of the present invention for achieving the first object, if the magnification of the projection optical system is xcex2 (e.g., xcex2=⅕, or xc2xc), the width of the local illumination area on the mask in the scanning direction is LR, and the width of the projection area (the similar area with respect to the local illumination area) of the pattern of the mask by the projection optical system in the scanning direction is LW, the widths LR and LW are in the following relation:
LR=(1/xcex2)xc2x7LWxe2x80x83xe2x80x83(1)
If the scanning speed of the substrate is VW, the scanning speed of the mask is VR, the speeds VR and VW are in the following relation:
VR=(1/xcex2)xc2x7VWxe2x80x83xe2x80x83(2)
When utilizing the light source for emitting continuous light, if the illuminance of the light on the substrate is Q, and the sensitivity (corresponding to the optimum amount of exposure) of the photosensitive material on the substrate is P, the exposure time t necessary for obtaining an optimum amount of exposure at a point is expressed as:
t=P/Qxe2x80x83xe2x80x83(3)
From the equations (1) and (2), the exposure time txe2x80x2 at a point on the substrate when the substrate is shifted at the speed VW with respect to the projection area of the mask pattern having the width LW is expressed as:
txe2x80x2=LW/VWxe2x80x83xe2x80x83(4)
Therefore, in order to make the exposure time t of the equation (3) equal to the exposure time txe2x80x2 of the equation (4), the following equation needs to hold:
xe2x80x83P/Q=LW/VW, i.e., Pxc2x7VW=LWxc2x7Qxe2x80x83xe2x80x83(5)
That is, in order to impart an optimum amount of exposure to the substrate in accordance to the sensitivity P of the photosensitive material, it is necessary to determine the width LW of the projection area, the illuminance Q of the light on the substrate and the scanning speed of the substrate VW in accordance with the sensitivity P so as to satisfy the equation (5). Then, in the present invention, while aiming at the equation (5), at least one of the width LW, the illuminance Q and the speed VW is made variable to impart an optimum amount of exposure to the substrate in accordance with the sensitivity P of the photosensitive material. Therefore, even though the sensitivity P of the photosensitive material is changed, an optimum amount of exposure can be imparted to the substrate.
When the equation (2) is substituted into the equation (5), the scanning speed VR of the mask is expressed as:
VR=LWxc2x7Q/(xcex2xc2x7P)xe2x80x83xe2x80x83(6)
Accordingly, when the width LW of the projection area, the illuminance Q and the magnification xcex2 are constant, the scanning speed of the mask is changed reasonably in accordance with the sensitivity P of the photosensitive material. Generally, in scanning type exposure apparatuses for manufacturing semiconductors, the projection optical system is the reduction type. That is, the magnification of the projection optical system xcex2 is less than 1. Therefore, as is apparent from the equation (2), the scanning speed VW of the substrate is faster than the scanning speed VR. Then, when the upper limit VRmax (maximum speed of the mask stage) of the scanning speed of the mask is less than 1/xcex2 times the upper limit VWmax (maximum speed of the substrate stage) of the scanning speed of the substrate, i.e., VRmax less than VWmax holds, the mask rather than the substrate easily reaches the upper limit. Accordingly, as the scanning speed VR of the mask needs to be set to equal to or less than the upper limit VRmax inevitably, the following relation holds from the equation (6). When the following equation (7) holds, the scanning speed VW of the substrate will not exceed the upper limit VWmax.
VR=LWxc2x7Q/(xcex2xc2x7P)xe2x89xa6VRmaxxe2x80x83xe2x80x83(7)
In order to impart an optimum amount of exposure to the substrate in accordance with the sensitivity P of the photosensitive material in consideration of the upper limit VRmax of the scanning speed of the mask, it is necessary to determine the width LW of the projection area, the illuminance Q of the light on the substrate and the scanning speed VW of the substrate.
In conventional scanning exposure apparatuses, only the scanning speeds VW and VR of the substrate and mask are made variable. Therefore, depending on the type of photosensitive material, there is a case that the scanning speed VW of the substrate determined from the equation (5) in accordance with its sensitivity does not satisfy the equation (7). Especially, when utilizing a photosensitive material with a high sensitivity (the value of the sensitivity P is small), the value of the left side of the expression (7) becomes large and the scanning speed VR of the mask might exceed the upper limit VRmax.
Then, in the present invention, according to the upper limit VRmax, at least one of the width LW of the projection area and the illuminance Q is made variable and the scanning speed VW of the substrate and at least one of the width LW are determined in accordance with the sensitivity P of the photosensitive material so as to satisfy the expressions (5) and (7). For example, in a photosensitive material with a high sensitivity, the width LW of the projection area of the mask pattern (i.e., the width of the local illumination area on the mask) is narrowed, or the illuminance Q of the light on the substrate is decreased. Therefore, even in such a highly sensitive photosensitive material, the scanning speed VR of the mask will not exceed the upper limit VRmax and an optimum amount of exposure can be imparted to the substrate. At this time, when the scanning speed VR of the mask is set to the upper limit VRmax and the scanning speed VW is set to xcex2xc2x7VRmax, the throughput becomes preferable while an optimum amount of exposure is imparted to the substrate.
On the other hand, when utilizing a photosensitive material having a low sensitivity (the value of the sensitivity P is large), the value of the left side of the expression (7) becomes small. Therefore, even though only the scanning speeds VW, VR of the substrate and mask are made variable, the scanning speed VR becomes slow but will not exceed the upper limit VRmax and an optimum amount of exposure can be imparted to the substrate. However, the decrease of the scanning speed of the mask (substrate) leads to lowering of the throughput. Therefore, even when the photosensitive material with the low sensitivity is utilized, it is desirable to make one of the width LW of the projection area and the illuminance Q variable. That is, in the photosensitive material with the low sensitivity, while the scanning speed VR of the mask is maintained to the upper limit VRmax, at least one of the width LW and the illuminance Q should be determined in accordance with the sensitivity P so as to satisfy the expressions (5) and (7). At this time, the width LW of the projection area of the mask pattern is widened or the illuminance Q of the light on the substrate is increased. Accordingly, even in the photosensitive material with the low sensitivity, an optimum amount of exposure can be imparted to the substrate while the lowering of the throughput is prevented.
The above description is directed to the case in which VRmax less than VWmax/xcex2 holds, but when VRmaxxe2x89xa7VWmax/xcex2 holds, the width LW of the projection area, the illuminance Q and the scanning speed VW of the substrate should be determined so as to satisfy both the expression (5) and the following expression (8):
VW=LWxc2x7Q/Pxe2x89xa6VWmaxxe2x80x83xe2x80x83(8)
When the expression (8) holds, the scanning speed VR of the mask never exceeds the upper limit VRmax. Also, even though all three conditions of the width LW, the illuminance Q and the speed VW are not made variable, it is sufficient to determine one or two variable conditions to satisfy the expressions (5) and (8) such that the scanning speed VW of the substrate will not exceed the upper limit VWmax and the throughput is not lowered.
Also, in order to achieve the second object of the present invention, a third apparatus of the present invention has a light source for emitting a light beam, an illumination optical system for illuminating a local area on a mask with the pulsed light from the light source and a projection optical system for projecting the image of the pattern of the mask within the local area to a substrate with a photosensitive material applied thereto. The image of the pattern of the mask is scanned and exposed on the substrate by synchronously shifting the mask and the substrate in a predetermined scanning direction perpendicular to the optical axis of the projection optical system. In this embodiment, the width of the projection area of the pattern of the mask by the projection optical system (a similar area with respect to the local illumination area on the wafer) in the scanning direction is set to an integer multiple of the distance by which the substrate is shifted relatively with respect to the projection area of the pattern of the mask for a period of the light emission of the light source.
As above, in this apparatus, e.g., in FIG. 13A, the width xcex2xc2x7L of the projection area (246P) of the pattern of the mask on the substrate (215) by the projection optical system is n times the distance xcex94L by which the substrate is shifted for the period of the light emission of the light source in the DW direction. That is, the following equation hold:
xcex2xc2x7L=nxc2x7xcex94L.
In this case, a position on the substrate on which an edge of the projection area (246P) is located when a light emission from the light source is done is a point P1 and the energy imparted to each positions on the substrate for a light emission is assumed to be xcex94E. Then, an energy of xcex94E/2 is imparted to the point P1 on the edge of the projection area (246P) at the time of a light emission. Therefore, the total energy of EP1 is imparted to the point P1 is as follows:
EP1=2xc3x97xcex94E/2+(nxe2x88x921)xc3x97xcex94E=nxc3x97xcex94E.
Also, with respect to a point P2 located slightly inside the edge of the projection area (246P), n light emissions are carried out while the point P2 is located within the projection area (246P). Namely, an energy of nxc3x97xcex94E is imparted to the point P2. As a result, the energy of nxc3x97xcex94E is imparted to each position on the substrate, so that unevenness of the illuminance does not occur.
On the other hand, in FIG. 13B, the width of the projection area (246P) of the pattern of the mask on the substrate in the DW direction is xcex2xc2x7L1 and 3.5 times the distance xcex94L by which the substrate is shifted in the DW direction for the period of the light emission of the light source. In this case, when the position on the substrate on which an edge of the projection area (246P) is located is Q1, an amount of energy imparted to the point Q is 3.5xc3x97xcex94E. Also, an amount EQ2 of energy imparted to a point Q2 located slightly inside the edge of the projection area (246P) is 4xc3x97xcex94E while an amount EQ3 of energy imparted to a point Q3 located slightly outside the projection area is 3xc3x97xcex94E. Therefore, the amount of energy imparted to each position on the substrate is varied within the range of 3xc3x97xcex94E to 4xc3x97xcex94E, thereby causing unevenness of the illuminance.